1. Field of the Invention
The present invention relates to a method and apparatus for purifying exhaust gases from lean-burn engines, in particular for controlling the amount of NO.sub.x and soot after a diesel engine.
2. Brief Description of the Prior Art
Diesel engines are more efficient than gasoline engines and emit lesser amounts of greenhouse gas. However, their emissions contain large amounts of nitrogen oxides (hereunder sometimes abbreviated NO.sub.x) and particulates (hereunder sometimes called soot). A diesel engine can be operated to emit less NO.sub.x or soot, but there is trade-off between the amount of soot and NO.sub.x. For example, if the engine is operated to reduce the amount of soot in the exhaust gases, the amount of NO.sub.x increases.
Diesel engines operate under lean-burn conditions. As a result, the exhaust gas has a high concentration of oxygen so that conventional three-way catalyst and oxygen sensor technology used with stoichiometrically fueled gasoline engines cannot be used for treating diesel exhaust gases. A number of so called lean-NO.sub.x catalysts have been developed which are selective for NO.sub.x reduction with organic chemical reductants. These catalysts have a relatively narrow temperature window in which they are selective for NO.sub.x reduction; above which, the reductant is oxidized without effective NO.sub.x control. For high efficiency NO.sub.x reduction, in addition to having an effective temperature window, the catalysts also require that the reductant be present in a certain molar ratio to the NO.sub.x.
The temperature of the exhaust gases from a diesel engine in transient operation, such as in a vehicle, varies from about 100 to 700.degree. C. Until now, there has been no practical converter capable of NO.sub.x reduction over that range of normal operating conditions. Initially there is a problem in bringing a selected lean-NO.sub.x catalyst to a temperature within the temperature window that it is selective for NO.sub.x reduction and then there is problem in preventing it from being overheated. There is also a problem in providing the chemical reductant in the right proportion with respect to the NO.sub.x.
Suitable reductants with lean-NO.sub.x catalysts are hydrocarbons, oxygenated organic compounds or carbon monoxide. Additional hydrocarbons or other reductant must be provided with lean-NO.sub.x catalysts as the amount of hydrocarbons in diesel exhaust is low.
Another NO.sub.x removal technology, which has been used for diesel exhaust aftertreatment, includes the selective catalytic reduction of NO.sub.x with a nitrogen containing compound such as urea or ammonia. Like the lean-NO.sub.x catalysts, the known catalysts for selective catalytic reduction of NO.sub.x (hereunder sometimes abbreviated SCR catalysts) provide for effective removal of NO.sub.x within some temperature window and with a sufficient amount of ammonia or other nitrogen containing reductant added to the exhaust gas. The temperature window for efficient operation of a SCR catalyst is typically wider than for a lean-NO.sub.x catalyst, yet there are no SCR catalytic systems that can provide for high NO.sub.x removal efficiency over the entire range of diesel exhaust temperatures.
A conventional catalytic converter for automotive exhaust aftertreatment includes a catalyst supported on a ceramic or metallic block or monolith with a plurality of straight, open channels for gas passage. In a conventional converter, the temperature inside the catalyst bed follows the temperature of the exhaust gases with some time delay. The temperature of the catalyst bed may then rise above the temperature of the exhaust gases since the reduction of NO.sub.x is an exothermic reaction. The exhaust gas parameters change quickly, with engine load and speed or during vehicle acceleration and deceleration, so that the temperature of the lean-NO.sub.x or SCR catalyst in the catalyst bed may fit the temperature window for NO.sub.x reduction for a while, but this favorable condition is not maintained long. Because the exhaust gases change in temperature with engine operating parameters, the catalyst quickly becomes overheated above the temperature window at which it is effective for NO.sub.x reduction, or overcooled below that window.
At low temperatures typical for low load mode of engine operation, the lean-NO.sub.x and SCR catalysts do not provide for appreciable NO.sub.x conversion. Hence at low temperatures, some of the hydrocarbons used as reductants over lean-NO.sub.x catalysts or the nitrogen containing compounds (e.g., ammonia or urea) used as reductants over SCR catalysts may be emitted with the exhaust gases, increasing the environmental hazard. At high temperatures, the hydrocarbon reductants over lean-NO.sub.x catalysts or nitrogen containing compounds over SCR catalysts quickly react with oxygen thus reducing the process selectivity for NO.sub.x reduction.
To some limited extent, the temperature of the exhaust gases can be controlled before or inside the converter. For example, a cooler can be installed in the exhaust pipe before the converter to control the temperature of the exhaust gases during engine acceleration. However, the cooler system is expensive, requires a suitable coolant and consumes energy. A heater system similarly adds to the cost and decreases engine efficiency.
Modern diesel vehicles are often supplied with catalytic or non-catalytic, filters capable of removing diesel exhaust particulates or soot. A popular commercial filter includes a ceramic monolith with a plurality of straight channels, opposite ends of which are opened or closed in checkerboard fashion. The particulates gradually accumulate on the filter walls. The filter can be regenerated by raising the temperature of the exhaust gases and burning the particulates off. The regeneration can be catalytically activated through the addition of metal oxides to the diesel fuel or by depositing an appropriate metal oxide catalyst on the filter ceramic substrate. For a non-catalytically activated filter, the typical temperature required for initial ignition of diesel particulates is in excess of 600.degree. C. This temperature can be reduced to about 350-400.degree. C. when the filter is catalytically activated. The temperature developed during the filter regeneration cannot be easily controlled as it depends on the amount of particulates accumulated. At high particulate capacity, the temperature can increase up to 1,200 to 1,400.degree. C. during regeneration, which may cause the ceramic support to break down or the catalyst washcoat to be destroyed. Filter regeneration could be substantially improved if the operating temperature of the soot filter was above the soot ignition temperature most of the time so that filter regeneration occurred continuously. However, this is not easily achievable in a conventional converter, where the filter temperature follows that of the exhaust gases, and, therefore, can be very low for extended periods of time, allowing the soot to accumulate on the walls of the filter.